1. Field of the Invention
The present invention relates generally to an improved apparatus and method for forming a sense resistor that is useful in an IC. As an example a current sense resistor is used in battery management circuits to determine the amount of current being supplied either to or from a rechargeable battery.
2. Description of the Related Art
Various types and mechanism for providing sense resistors have been known. For example an external resistor can be soldered on the board next to a packaged integrated circuit. This however requires several extra manufacturing steps and is inefficient.
Alternatively, the lead frame that the IC is attached to can be formed in such a manner so that a portion of the lead frame can be used as the sense resistor. An example of this is Smith et al., U.S. Pat. No. 5,534,788 which is incorporated by reference herein. As noted in Smith et al. a sense resistor having a low resistance is needed for measuring for cell current in a rechargeable battery pack for instance without affecting device operations. The Smith et al. system cuts down somewhat on the bulk of this resistor and cuts to some extent manufacturing costs of the individual circuits by including as part of the lead frame a sense resistor.
The inclusion of a sense resistor in the lead frame as proposed by Smith et. al has problems however, for several reasons. Particularly the requirement for specialized lead frames is costly. If the die layout or size changes a new customized lead frame must be manufactured requiring either a new stamping or a new cut out to produce this customized lead frame. If a etched lead frame is used this is also expensive and time consuming. It is difficult in the Smith et al. method to have a generic "one size fits all" lead frame.
Further the use of a lead frame itself takes up additional space that is not needed in many semiconductor applications.